For those wondering how this went: we had seven participants: five writers with varying levels of experience (from one game to more than ten), and two first-timers. We had a long period of discussion at the beginning, covering things we enjoyed / didn't enjoy in larp, goals and ways of writing them, goal vs situation vs event/crew-driven games, and specific design issues around small games.
After a break for coffee we pitched ideas. The second idea we discussed was "space pirates", which everyone was enthused by. Within an hour we had a bunch of ideas for a small game on the board, and after a break for lunch we gridded them up and assigned them to characters while refining them. That took us to our 4pm time limit, so we ended up not writing any characters. But we're all keen and have set a deadline of next weekend to get our initial sheets in. Hopefully a blurb will be appearing here in due course.